Remove IMMS#mUserManager, which is unused

With recent CLs [1][2],

  InputMethodManagerService#mUserManager

is no longer used.  We can safely remove it.

 [1]: I10cfc44465db110a89e7166ab7a60510e72533ea
      3cec57b9
 [2]: Ie1e6bab0f9a5d2801ad57c89c23f0798276d8aa2
      f04121f0

Bug: 234882948
Test: presubmit
Change-Id: I8de940561a4ad464a20b1d8f208216ef0bcde4db